1. Field of the Invention
The present invention relates to a traction control system or a slip control system of a motor vehicle, in particular to a traction control system which is applied a motor vehicle having a four-wheel steering system (4WS) which steers rear wheels based on vehicle driving conditions such as a vehicle speed, a front wheel steering angle and a yaw rate.
There is known a conventional traction control system for a motor vehicle which prevents a decrease in acceleration ability caused by the slip which is caused by an excessive driving torque of the driven wheel in an accelerating operation and the like. In order to do so, such slip control system detects a slip value of the driven wheel and controls an engine output and braking force so that the slip value of the driven wheel becomes less than a target slip value which is provided based on friction coefficient of the road surface. The engine output is decreased by the engine output control and the braking force is increased by the braking force control. In the engine output control, a throttle opening in a throttle valve of an engine is controlled. Such conventional traction control system for the motor vehicle is disclosed in for example Japanese Patent Laid-Open No. 2-252930 and Japanese Patent Laid-Open No. 3-109161.
On the other hand, there is known a four-wheel steering system (4WS) which steers rear wheels based on a difference between a target yaw rate and an actual yaw rate of the motor vehicle.
In such four-wheel steering system, the yaw rate increases based on driving conditions of the motor vehicle. The reason why the yaw rate increases may be considered as follows. That is, firstly, the vehicle has an over-steering condition since road surface friction coefficient .mu. is small and therefore a necessary cornering force of the rear wheels cannot be established by a steering operation of rear wheels. Secondly, the front engine rear drive vehicle (FR vehicle) has an over-steering condition since the FR vehicle has a high slip ratio in rear wheels and therefore a necessary cornering force of the rear wheels cannot be established. On the other hand, the rear wheel steering angle increases based on driving conditions of the motor vehicle. The reason why the rear wheel steering angle may be considered as follows. That is, the vehicle has an over-steering condition since a necessary cornering force of the rear wheels cannot be established when the rear wheel steering angle has the same phase as the front wheel steering angle during the vehicle traveling in a corner.
As a result, under the above-mentioned three driving conditions, when the engine output is decreased and/or the braking force of the wheels is increased by the traction control system independently from the four-wheel steering system, the above-mentioned driving conditions are enhanced and therefore it becomes difficult to keep both of stability and steering ability of the vehicle in good conditions.
How to control cooperatively the traction control system and the four-wheel steering system is not so well known such that only Japanese Patent Laid-Open No. 4-66359, and therefore there are many unsolved problems.